How do I go about getting a Research Assistant (RA) position for the summer?
You should contact the faculty member(s) with whom you would like to work, asking if they need an RA for the summer. A number of faculty members have large external grants which allow them to fund one or more RAs. Others have UCSD funding for this purpose. This is a great way to build ties to faculty and to learn skills that will be useful to your academic progress.
